1. Field of the Invention
This invention relates to a process for the continuous diazotization of water-soluble polymeric amines. In a preferred embodiment, it applies this process to the preparation of azobond containing polymeric materials useful as pharmaceuticals.
2. Discussion of the Prior Art
The following United States Patents are commonly assigned herewith and are believed to be representative of references that relate to the present invention--U.S. Pat. Nos. 3,920,855, Dawson et al; 4,190,716, Parkinson et al; and Re 30,362, Gless et al.
These references disclose that a polymeric aromatic amine can be reacted with nitrous acid at low temperature to form a polymeric diazonium salt, and that this diazonium salt can thereafter have various aromatic groups attached so as to form polymeric dyes, (See Dawson et al and Gless et al); polymeric pharmaceuticals, (See Parkinson et al); and the like. It is also known to substitute other diazotizing moieties for nitrous acid or to use other coupling agents such as materials of the structure ##STR1## wherein R is an alkyl, alkoxy, alkylamine, etc; in place of the aromatic coupling groups.
In the course of the diazotization steps involving a polymeric aromatic amine, the polymer will go through a state in which both diazo groups and amines are present on the same and on neighboring polymer molecules: ##STR2##
The diazo groups can react with amines to form a triazine crosslink which will render the polymer insoluble. This triazine-forming reaction is reversible and, if the polymer has not contracted to a highly condensed phase (a precipitate), diazotization will proceed and a fully soluble polymeric diazonium salt will result. However, if the polymer forms a condensed phase, the probability of polymer-polymer reactions involving the highly reactive diazo group becomes so high that a multitude of crosslinks form, many of which are not reversible (eg), ##STR3## In this case, the polymer is permanently crosslinked and insoluble.
Accordingly, if one desires to produce soluble derivatives of polymeric diazonium salts, for example, polymeric azo dyes or polymeric azo group-containing pharmaceuticals, it is critical to diazotize in such a fashion that a condensed polymer phase never forms. This operation can be successfully conducted on a small scale (up to 1-5 liters in volume) by means of rapid addition of greater than one equivalent of sodium nitrite solution to an acidic polymeric amine solution, with thorough agitation. However, on a larger scale, it becomes impractical, often impossible, to duplicate this process. In the case of a polymeric amine with a DP=1400, if the sodium nitrite addition takes over 5-10 seconds, some degree of permanent crosslinking is inevitable; clearly, mixing hundreds of gallons of solutions in less than 5 seconds is not generally feasible.